Process and an apparatus for extracting fine droplets of paint from paint-spraying units

ABSTRACT

The invention relates to a process and an apparatus for extracting droplets of paint from the fine spray or mist in spray booths when sheet metal members or the like are sprayed, without the paint droplets adhering. A calcium chloride solution of 0.01 to 0.5% by weight and having a pH value of 8 to 9.5 serves as washing fluid. To perform this process, the spray booth and the like is followed by a settling or separating vessel in which the coagulated paint droplets floating up with air are separated from the thus regenerated wash solution which can be recycled.

BACKGROUND OF INVENTION

Paint-spraying is carried out on a large scale in the automobileindustry or in all other industries producing apparatus and equipment,where sheet metal members have to be painted in large quantities, as fordomestic appliances, electrical appliances, machine panelling or thelike. In this technique, the coating medium is atomized, for instancewith the aid of a spray gun, and the spray of very fine droplets isdirected onto the article to be painted. Since a high quality finishrequires that the article be coated evenly, also in the edge areas, partof the spray of droplets goes past the article to be painted and strikesa wall of the spray booth. The fine spray or mist of paint droplets inthe air of the spray booth not only reaches the walls of the spray boothitself, but also gets into the drain pipes and the following units. Itis therefore necessary to largely prevent or remove these deposits ofpaint droplets on the booth walls, in the drain pipe connections and thefollowing units. To this end, the respective walls or the like aregenerally supplied with water, so that a closed film of water runs downthe walls, taking the droplets with it as they strike the wall. Thepaint-laden fluid collecting in the lower zone of the spray booth thenpasses into a separating vessel to separate the paint from the aqueousphase and to recycle this aqueous phase to the spray booth.

If water without any additives is used for extracting the paintdroplets, then this leads to particles of paint reaching the spray boothwalls through the water film and adhering thereto or to the drain pipeconnections and the following units; it may also lead to plugging of thenozzles or the like with whose aid the aqueous phase recycled to thespray booth is distributed over the walls.

To solve this problem, one has alrady used salts of sulfonic acids (U.S.Pat. No. 2,208,657), reaction products of starch with formaldehyde(German Pat. No. 836 759), polyvinyl compounds and derivatives ofhigh-polymer carbohydrates (German Auslegeschrift No. 10 17 957),polyamines, polyacrylamides, polyacrylic acids, polymethacrylic acidsand polyoxyethylenes (French Pat. No. 15 13 413), oxalic acid (GermanOffenlegungsschrift No. 22 00 807)or also salts of carboxylic acid(German Offenlegungsschrift No. 23 47 068). In such spray units,droplets of paints capable of being thinned with water were extractedfrom the fine spray or mist using as aqueous phase solutions of saltshaving a monovalent, bivalent or trivalent cation. Calcium, barium,manganese, zinc, copper, lead, cadmium and cobalt combined with theanions sulfate, chloride, acetate, tartrate, citrate, phosphate ornitrate were used as bivalent cations. In the prior art process, thesalts of bivalent metals had a concentration of 0.4 to 2% by weight insuch wash solutions, attention having to be paid that the pH value ofthis wash solution was not less than 2 and not more than 10. With theaid of this wash solution, the paint droplets coagulated. The coagulatecould be separated from the aqueous phase, i.e. from the wash solution.Special control of the pH value is not necessary in this form of priorart.

In the case of all these prior art measures for paints incapable ofbeing thinned with water, the paint coagulate collects at the bottom ofthe separating vessel. German Auslegeschrift No. 24 33 193 has disclosedthat for paints capable of being thinned with water, a concentratedcalcium chloride solution of about 40% by weight and having a density ofabout 1.43 g/cm³ be used as washing fluid. Owing to the high density ofthe aqueous phase, the paint coagulate floats to the top of theseparating vessel and can be skimmed off the aqueous phase. This priorart process has two major drawbacks, namely, the costs of the largeamounts of calcium chloride required are considerable and, in addition,a highly concentrated calcium chloride solution is a corrosive fluidwhich acts on the separating vessel, the relative piping, the boothwalls and the like. It is not possible for this prior art process to beused for paints incapable of being thinned with water.

Finally, liquid coagulating agents or flocculating agents are on themarket which have to be strongly alkaline, so that such wash solutionshave to be expected to have a pH value of at least 11 in order for thepaint droplets to be coagulated. These liquid coagulating agents areused both for paints containing metal pigments (metal effect or metallicpaints) and for paints containing colour pigments ("Uni" paints). Thedrawback with these prior art coagulating agents, particularly in thecase of metallic paints, is the high pH value which leads to the metalpigments, e.g. aluminum bronze, decomposing or dissolving and generatinghydrogen. For this reason the precipitated paint coagulate is frothy anddischarge thereof by the conventional discharging means, e.g. scraperbands, cannot be effected or can only be effected in an unsatisfactorymanner. Apart from this, the generation of hydrogen represents anexplosion hazard which is not to be underestimated, particularly in viewof solvent vapour possibly being present. The high pH value of thisprior art liquid coagulating agent also leads to the paint itselfundergoing change or decomposing, the metal pigment being separated anddeposited and the paint base of the coagulate becoming less viscous,this impeding flotation of the coagulate. In this case the paintcoagulate floats up due to the wash solution, which contains droplets ofpaint and runs down the walls of the spray booths of the like,entraining air which attaches to the paint droplets in the form of smallbubbles and in this way causes the paint coagulate to float to the topof the thus regenerated wash solution. A further drawback encounteredwith this prior art process using strongly alkaline liquid coagulatingagent is that this wash solution cannot be drained off into themunicipal waste water system, but first has to be neutralized to a pHvalue of at most 9.5 in line with statutory regulations. This requiresconsiderable amounts of acid and the provisions needed for using andhandling these amounts of acid represent a handsome cost factor.

SUMMARY OF THE INVENTION

The invention relates to spraying shaped members with paint by atomizingthe paint mixed with metal pigments or colour pigments to form a sprayof droplets, directing the spray of droplets against the shaped memberto be painted, the paint droplets in that part of the spray passing thearticle to be painted being taken up in a film of liquid running downthe spray booth walls, the fluid laden with paint droplets beingcollected and conducted into a separating or settling vessel where thefloating coagulated paint droplets are separated from the fluid and thefluid is recycled, the improvement comprising the use of an aqueouscalcium chloride solution having a concentration of 0.01 to 0.5% byweight CaCl₂ and a pH value of 8 to 9.5 as fluid, flotation of thecoagulated paint droplets with air, separate discharge of the floatedpaint coagulate and the calcium chloride solution and recycling of thethus regenerated calcium chloride solution to the walls of the spraybooth. The invention further relates to an apparatus for performing saidprocess from a spray booth having feeding means for fluid in the upperpart so as to allow the fluid to flow down the walls in the form of ahomogenous film, means for gathering and collecting the fluid which hasflowed down and conducting the same into a settling or separatingvessel, and a settling or separating vessel having a discharge pipe forthe coagulated paint droplets in the upper area and a pipe for returningthe regenerated calcium chloride solution from the lower area of thesettling and separating vessel to the upper zone of the walls of thespray booth.

The object of the invention is to provide a process for extractingdroplets of paint, in particular of paint incapable of being thinnedwith water, from fine sprays or mist in paint-spraying units sprinkledwith calcium chloride solution as wash solution and for separating thepaint coagulate from the wash solution which is recycled to thepaint-spraying unit, it being achieved that the paint droplets coagulaterapidly and completely and it being avoided that the paint dropletsadhere to the walls of the spray booth, the drainage system or thereturn piping. The washing fluid containing paint coagulate can settlein a settling or separating vessel, whereupon the paint coagulatecollecting on top of the washing fluid is separated from the washsolution and the latter is recycled to the spray unit.

BRIEF DESCRIPTION OF DRAWING

The sole drawing schematically shows a simple apparatus in accordancewith the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

The above object is accomplished by using a washing solution containing0.01 to 0.5% by weight CaCl₂ and having a pH value of 8 to 9.5, said pHvalue being adjusted with the aid of a strong base. Sodiumm hydroxidesolution or potassium hydroxide solution is preferred as base. In thecase of metallic paints, the wash solution should preferably have aconcentration of 0.05 to 0.1% by weight CaCl₂ and a pH value of 8 to8.5. In the case of colour pigment paints, the wash solution shouldpreferably have a concentration of 0.2 to 0.5% by weight CaCl₂ and a pHvalue of 8.5 to 9.5. To prepare the wash solution employed according tothe invention, any CaCl₂ material is used. Owing to it being readilyavailable, an 80 wt. % CaCl₂ of industrial quality is preferred, whoseCaCl₂ content has, however, to be considered in adjusting theconcentration on the washing fluid.

It is important that the above-stated ranges of concentration of calciumchloride be observed in the wash solution. The coagulation may not besatisfactory below the aforesaid lower limits of concentration, whileabove the aforesaid upper limits nothing will be gained in the way theprocess according to the invention functions, but the costs for material(calcium chloride) and the danger of corrosion increase.

It is also important that the said pH value range be observed. Above apH of 9.5, it can happen that calcium hydroxide precipitates, whilebelow a pH of 8, the coagulation may not be sufficient in some systems.With the said pH values, the calcium chloride may partially hydrolyze inthe wash solution laden with paint droplets, possibly with the formationof calcium oxychloride or calcium hydroxide which could be effective asnuclei for the coagulation of the paint droplets.

As already mentioned above, the washing fluid running down the walls notonly takes along the paint droplets from the fine spray or mist, butalso air which forms small bubbles on the paint particles or dropletswithin the aqueous phase of the wash solution and causes them to float.This flotation can be promoted by blowing air into the settling vessel.

The process according to the invention is suitable for the extraction ofall sorts of paints, as are used in industry, e.g. alkyd resins,acrylate resins, epoxy resins, melamine resins, polyester resins and thelike. These paints may be ones which are capable of being thinned withwater--as are stoving lacquers, such as polycarboxylic acid inneutralized form--or incapable of being thinned with water. However, theprocess according to the invention affords especial advantages withpaints which are incapable of being thinned with water.

The coagulation of the paint droplets is generally effected with theagency of an electrolyte. In this case, the calcium chloride of the washsolution also has the coagulating effect of an electrolyte on the resindroplets. The coagulate separates rapidly and completely from the liquidphase and therefore can readily be separated from the same.

Bactericides, corrosion inhibitors and similar additives may be added tothe wash solution, particularly for the extraction of colour pigmentpaints, so as thus to extend the time over which the calcium chloridesolution can be used.

With reference to the drawing which schematically depicts a simpleembodiment of the present apparatus, there is shown a spray booth havinga rear wall 1 with a gathering or collecting vessel 4 at the bottomthereof. A distributing pipe 2 is provided near the upper part of therear wall 1 of the spray booth, the distributing pipe 2 having apertures3 facing the rear wall 1 in order to distribute liquid from the pipe 2so that it will run down the rear wall 1 as shown. A distributing duct 5including spraygun arms 6 is arranged adjacent the bottom of thecollection vessel 4. A recycling line 13 having a pump 14 along itslength branches off from the bottom of the collection vessel 4 and leadsto the distributing pipe 2. Such distributing pipe 2 is also providedwith a fresh water duct 16. A collection vessel discharge means isprovided at a suitable height of the collection vessel 4.

In operation, an article 8 to be painted is coated with paint by the useof a spraygun 7. As is usual, the spray cone of the gun 70 exceeds thesize of the article 8 to be painted to some extent so as to insure thatthe peripheral edges of the article 8 receive a sufficient quantity ofpaint. The consequence of such overspraying is that within a zone 9around the article 8, unused sprayed paint would reach the rear wall ofthe spray booth. In order to prevent paint particles from sticking onthe rear wall 1 of the spray booth, a fluid film 10 is uniformlydistributed by the apertures 3 of the distributing pipe 2 so as to flowdown the entire rear wall 1 of the spray booth so as to take up to anincreasing extent the paint droplets 10' and 10".

In the fluid film, provided the flow period is long enough, there isachieved a complete coagulation of the paint droplets into readilyseparable aggregates. The wash fluid flowing down the rear wall 1 of thespray booth is in the instant invention a calcium chloride solution, aspointed out above. The calcium chloride causes the paint droplets tocoagulate. The wash fluid laden with paint droplets and with coagulatedpaint droplets in particular, collects in the vessel 4 and separatesinto a lower aqueous phase 11 containing calcium chloride and optionallysmall quantities of non-coagulated paint droplets, and into a upperorganiz phase 12. The separation of the aqueous phase from the organicphase, and in particular the forming into a "cream" of the coagulatedpaint droplets in the organic phase, are assisted and accelerated by airintroduced through the distributing duct 5 and nozzle arms 6. Theaqueous phase can be removed from the vessel 4 continually orintermittently via the recycling line 13, and thereby recycled to thedistributing pipe 2 with the aid of the pump 14.

The organic phase substantially consisting of coagulated paint dropletsis removed through the discharger means 15 and is optionally processedfor reuse in paint spraying. If necessary, fresh calcium chloridesolution can be fed to the distributing pipe 2 via supply line 16, andhence into the process cycle.

The process according to the invention is illustrated further with theaid of the following Examples:

EXAMPLE 1

In a settling vessel of a paint-spraying unit for metal-effect paints onthe basis of polyacrylate, 200 kg calcium chloride were dissolved in 200m³ water to give a solution of 0.1% by weight. As the calcium chloridewas of industrial quality, the concentration of the wash solution wasactually 0.080% CaCl₂. This calcium chloride solution was adjusted to apH value of 8 under agitation by means of a sodium hydroxide solution ofabout 50% by weight.

This wash solution was then pumped to the upper edge of the spray boothwalls which were to be water-covered, from where the wash solutionflowed down as a continuous film. This water film took up the finedroplets of paint passing the article to be painted and transported thepaint droplets into the settling vessel where the droplets coagulatedand floated up. The paint coagulate was skimmed off and is deposited orburned. The regenerated wash solution freed of paint was recycled to thespray booth. By adding the respective amounts of calcium chloride and/orbase at the proper time, the concentration of calcium chloride and thepH value were maintained at the above-stated values.

After having been in use for four months, the wash solution wascompletely clear and could be drained into the municipal waste watersystem without any further steps being taken. No appreciable depositionof coagulate or generation of gas was observed. There was hence nofrothing in the case of metallic paints. No difficulties were presentedin discharging the paint coagulate.

COMPARITIVE EXAMPLE A

Example 1 was modified in that instead of calcium chloride solutionhaving the above pH value, a commercial liquid coagulating agentcomprising a strong inorganic base as chief constituent ("Isogel 223" ofMessrs. Kluthe, Heidelberg and "VR 7803-1" of Messrs. Henkel,Dusseldorf) was fed into the settling vessel, in accordance with thesuppliers' instructions, until the pH value was above 11.

After having been in use for one month, the wash solution undercomparison was distinctly cloudy. Before the wash solution could bedischarged into the municipal waste water system, it was necessary forsludge to be removed and for the solution to be neutralized.

COMPARITIVE EXAMPLE B

Comparitive Example A was modified in that the amount of coagulatingagent introduced into the settling vessel was only sufficient for thewash solution to have a pH value of 8.5 to 9.5. With this wash solution,neither did the paint particles coagulate nor could the paint dropletsbe prevented from adhering to sliders, piping, nozzles for recyclingliquid to the walls of the spray booth or the like.

EXAMPLE 2

Example 1 was modified in that a polyacrylate filled with colour pigmentwas used. The wash solution had a concentration of 0.2% by weight CaCl₂and a pH value of 8.5.

EXAMPLE 3

A metal-effect paint on the basis of an alkyd resin was used in the unitof Example 1. A calcium chloride solution of 0.1% by weight and having apH value of 8.5 was used as wash solution.

EXAMPLE 4

Example 3 was modified in that an alkyd resin paint filled with a colourpigment was used. A calcium chloride solution of 0.5% by weight andhaving a pH value of 9.5 was used as wash solution.

EXAMPLE 5

In this Example, a metal-effect paint on the basis of an epoxy resin wasused. The wash solution was a calcium chloride solution of 0.09% byweight and having a pH value of 8.3.

EXAMPLE 6

Example 5 was modified in that the epoxy metal contained a colourpigment instead of the metal pigment and the wash solution had aconcentration of 0.4% by weight CaCl₂ and a pH value of 9.3.

EXAMPLE 7

A polyester metal-effect paint was used in accordance with Example 1. Acalcium chloride solution of 0.05% by weight and having a pH value of 8served as wash solution.

EXAMPLE 8

Example 7 was modified in that a wash solution comprising a 0.3% byweight calcium chloride solution--pH value 9-- was used for a polyesterresin paint filled with a colour pigment.

EXAMPLE 9

A paint capable of being thinned with water and filled with colourpigment, said paint being on the basis of a neutralized polycarboxylicacid was used in the spray unit of Example 1. In this case, a calciumchloride solution of 0.48% by weight and having a pH value of 8.1 servedas wash solution.

It proved that with the wash solutions according to the invention, goodcoagulation of the paint droplets and flotation of the coagulate wasachieved for all the various paints as were employed in Examples 1 to 9.To promote flotation of the coagulate, air could be blown in throughnozzles in the separating and settling vessel. In principle, however,the air entrained by the washing fluid is sufficient for flotation ofthe coagulate.

What we claim is:
 1. An apparatus for spraying shaped members with paintby atomizing the paint mixed with metal pigments or color pigments toform a spray of droplets, directing the spray of droplets against theshaped member to be painted, from a spray booth having feeding means foraqueous calcium chloride solution in the upper part so as to allow thesolution to flow down the walls in the form of a homogenous film, meansfor gathering and collecting the solution which has flowed down andtaken up coagulated paint droplets and conducting the same into asettling or separating vessel, and a settling or separating vesselhaving a discharge pipe for the coagulated paint droplets in the upperarea and a pipe for returning the regenerated calcium chloride solutionfrom the lower area of the settling and separating vessel to the upperzone of the walls of the spray booth, said settling vessel beingprovided with nozzles for injecting air to promote the flotation of thecoagulated paint droplets.